The erythrocyte sedimentation rate (ESR) is seldom the sole clue to disease in asymptomatic persons and is not a useful screening test. When the rate is increased, a careful history and physical examination will generally disclose the cause. An unexplained increase in the ESR is generally transitory and seldom due to serious disease. The test is most useful in diagnosing temporal arteritis and monitoring the patient's response to treatment. The test has little diagnostic value in rheumatoid arthritis but may be useful in monitoring disease activity when clinical findings are equivocal. The ESR is often normal in patients with cancer, infection, and connective tissue disease and is therefore of little use in excluding these diseases in patients with vague complaints.